High frequency supercapacitors and methods of making same

ABSTRACT

High-frequency supercapacitors that can respond at kilohertz frequencies (AC-supercapacitors). The electrodes of the AC-supercapacitors include edge oriented graphene (EOG) electrodes or carbon nanofiber network (CNN) electrodes. The EOG electrodes are formed by utilizing a plasma and feedstock carbon gas to carbonize cellulous paper and deposit graphene implemented in one step. The CNN electrodes are formed by pyrolyzing a carbon nanofiber network utilizing a plasma.

RELATED PATENTS/PATENT APPLICATIONS

The application claims priority to U.S. Patent Application No.62/491,097, entitled “Ultrahigh-Rate Supercapacitors With LargeCapacitance Based On Edge Oriented Graphene Coated Carbonized CellulousPaper As Flexible Freestanding Electrodes,” filed on Apr. 27, 2017,which application is commonly assigned to the Applicant of the presentinvention and is hereby incorporated herein by reference in its entiretyfor all purposes.

GOVERNMENT INTEREST

The invention was made with governmental support under Grant No. 1611060awarded by the National Science Foundation. The government has certainrights in the invention.

TECHNICAL FIELD

The present invention relates in general to ultrahigh-rate (orhigh-frequency, HF) supercapacitors, including HF-supercapacitors withlarge capacitance based on edge-oriented graphene coated carbonizedcellulous paper or carbonized cellulose nanofiber aerogel (or network)or carbonized polymer nanofiber network as electrodes.

BACKGROUND OF THE INVENTION

With billions of pieces sold every year, aluminum electrolyticcapacitors (AECs) are a ubiquitous component used in any power relatedcircuit design for current ripple filtering in power conversion andpower conditioning for electronic devices and power systems. They mayalso be used as fast energy storage for environmental pulse energyharvesting or as pulse power generators for strobe applications.Billions of pieces of AECs are sold per year and are utilized inconsumer, medical devices, defense devices, the telecommunicationsindustry, and the renewable energy markets. Even though AECs are sowidely used, they still come with their share of problems. Their bulkysize, polarity sensitivity, large equivalent series resistance (ESR) andhence high loss and high thermal generation, low temperature rating, andpoor lifetime, are some of the pain points customers face.

Supercapacitors, also called as electrochemical capacitors (ECs),electric double layer capacitors (EDLCs), or ultracapacitors, have acapacitance density larger than AECs by several orders of magnitude andtherefore can store much more energy than AECs. They are being widelyinvestigated to further increase their energy density to be close tothat of batteries, so that they function as independent energy source orin supplement with the low power batteries. As energy sources,conventional electric double layer based supercapacitors have a chargeand discharge rate (frequency) limited up to approximately 1 s (1 Hz).In other words, they work under direct current (DC). They will behavemore like a resistor but not a capacitor at higher frequencies.Therefore, although these conventional supercapacitors can have a muchsmaller volume size than AECs due to their much large capacitancedensity, they cannot play the roles of AECs that work at much higherfrequencies for ripple current filtering, decoupling, high-frequencypulse energy storage, short pulse generation, and other functions.

A huge performance gap exists between conventional supercapacitors andAECs. Supercapacitors provide much larger capacitance density than AECs,but the latter can respond at tens of kilohertz (kHz) frequencies. SinceABCs are commonly used in power modules and circuits for ripple currentfiltering and other functions, while their large size is becoming alimiting factor in downscaling the electronic appliances size orprofile, there are great needs and interests in developinghigh-frequency (HF) supercapacitors with a much reduced size to replaceABCS for filtering and other applications. For 60 Hz line-frequency,considering the harmonic frequencies after rectification, suchHF-supercapacitors should respond at hundreds to kilo hertz range. Inanother frontier, environmental energy harvesting through piezoelectric-or triboelectric-generator is promising for self-powered autonomoussensors and internet of things (IOT) technology, while the environmentalmechanical energy sources and hence the outputs from themicro-generators are typically pulsed currents with tens or hundreds Hz.They also require kHz HF-supercapacitors to efficiently store theharvested energy. In addition, HF-supercapacitors with their largecapacitance density and hence small volume, will also be useful forshort pulse (down to millisecond or sub-millisecond range) generationused in portable/wearable devices, implantable devices, or others.

Using miniature HF-supercapacitors to replace AECs will not only solvethe bulky size problem of AECs, but may also overcome other issuesrelated with AECs. AECs have the intrinsic positive/negative polarityproblem, while HF-supercapacitors are non-polar device. AECs have largeequivalent series resistance (ESR) and hence high power loss and highthermal generation, low temperature rating, and poor lifetime, whileHF-supercapacitors, if properly design, can have low ESR, low powerloss, long lifetime. They also could have high temperature rating. Withall these potential merits of HF-supercapacitors, now the question ishow to design HF-supercapacitors to be able to run at hundreds hertz(Hz) and even kilohertz (kHz) frequency range, considering that theconventional supercapacitors only runs at DC or below a few hertzfrequency.

To evaluate the response frequency of a capacitor, including asupercapacitor, the characteristic frequency ƒ₀ when its impedance phaseangle reaches −45° is commonly used, which defines the boundary betweencapacitance dominance and resistance dominance. Of the two categories ofsupercapacitors, pseudocapacitors are intrinsically slow even thoughfast pseudocapacitors were reported, while EDLCs in principle arecapable of charging and discharging within milliseconds, promising forkilohertz (kHz) HF-supercapacitors. The frequency response of these EDLCbased supercapacitor strongly depends on the nanostructure ofelectrodes, conductivity of the electrode material and the choice ofelectrolyte. In terms of the electrode nanostructure, reduction ofporosity and electrode thickness to accelerate electrolyte ionstransport is the key for high-frequency response. However, thisdiminishes the achievable capacitance.

Towards high-frequency response by partially sacrificing capacitance,vertically-oriented graphene (VOG) and carbon nanotube (CNT) ultra-thinfilm have been widely studied. Since very thin (˜1 μm to 0.1 μm)“active” materials were used on a thick metal substrate, they provided avery limited electrode areal capacitance density and volumetric densitywhen the current collector also included. For conventional sandwich-typecell configuration, practical consideration requires a fairly thickactive material layer to minimize the mass and volume penaltiesassociated with the inactive components that include separators,electrolytes, current collectors, and packaging. This is particularlytrue when multiple cells have to be serially packed together for a highvoltage rating. Further improving the areal capacitance to a new levelis in demand to develop compact HF-supercapacitors.

As a summary, AECs are a ubiquitous component used in any power relatedcircuit design for current ripple filtering in power conversion andpower conditioning for electronic devices, as well as for industrialpower supplies, solar and wind based renewable energy generation andmany others. They may also be used as fast energy storage forenvironmental pulse energy harvesting or as pulse power generators forstrobe applications. Bulky size, poor lifetime, large equivalent seriesresistance (ESR), and polarity sensitivity are some of the drawbacks ofAECs. There are critical needs to improve them. The applications and thecorresponding customer needs can be divided into segments such as: (1)for compact, low-profile and other space-demanding applications, the lowcapacitance density and hence bulky size of AECs is the acute pain pointthat must be addressed; (2) for space undemanding but power ortemperature demanding heavy-duty operations, the relative large thermalgeneration due to large ESR and high failure rate with limited lifetimeof AECs are often complained by customers, but lack of alternativetechnologies to fulfill their needs. Surge, peak or pulse current causedpolarity reverse also shortens the AEC lifetime; (3) in the emergingmarket of environmental energy harvesting for wearable electronics andinternet of things (IoT) sensors, fast energy storage is in demand toefficiently store the pulse energy scavenged from environmentalvibration, noise, or other sources. Compact size or slim/flexible formatand efficiency are critical requirements, which AECs cannot meet with.Voltage spike in these applications can also easily cause polarityreverse and hence damage to the polar AECs; and (4) high-frequencyand/or high-power pulse generation to drive actuators, for strobes orburst communication, or short pulse generation for wearable orimplantable devices to provide aid or support the smooth functions ofthe muscle, organs, or heart such as cardiac pacemaker, compact size orslim/flexible format and efficiency are critical requirements, whichAECs cannot meet with. On the other hand, when probably designed, theHF-supercapacitors can meet these application needs very well. Due tothe fact that miniaturization and other high performance are highlyvalued in today's market, there are great needs for suchHF-supercapacitors.

Since conventional supercapacitors can only run at very low frequency orquasi-DC condition, in the following, HF-supercapacitors are also calledas AC-supercapacitors or “AC-supercaps”. When there is no ambiguousness,they are also simply called as “cell”.

SUMMARY OF THE INVENTION

The present invention relates to AC-supercapacitors (also referred to as“AC-supercaps”) that runs at kHz frequencies and method of producingsame. These AC supercaps are much smaller and have much betterperformance than commonly used aluminum electrolytic capacitors (AECs).

In general, in one embodiment, the invention features a method of makingan AC-supercapacitor. The method includes selecting a materialcomprising polymer nanofiber network. The method further includesemploying a plasma to pyrolyze the polymer nanofiber network to create acarbon nanofiber network (CNN) electrode material. The method furtherincludes forming a CNN electrode from the CNN electrode material. Themethod further includes incorporating the CNN electrode in a cellpackage to form an AC-supercapacitor. The AC-supercapacitor is operablefor running at frequencies of at least 0.1 kHz.

Implementations of the invention can include one or more of thefollowing features:

The AC-supercapacitor can be operable for running at frequencies of atleast 1 kHz.

The step of employing the plasma can include utilizing a microwave or RFor DC plasma system.

The method can further include placing the material into a plasmachamber. The method can further include that, during the step ofemploying the plasma, flowing a gas in the plasma chamber at a pressureof less than 50 Torr and then generating the plasma.

The gas can include only one or more non-carbon based gases.

The gas can include comprises at least one carbon based gas.

The carbon based gas can be not used as a carbon feedstock gas in themethod.

The carbon based gas can be methane or acetylene.

The gas can further include H₂ or ammonia.

The polymer nanofiber network can include a cellulose polymer nanofibernetwork.

The cellulose polymer nanofiber network can be produced from a processcomprising microbial fermentation.

The cellulose nanofiber network can include a bacterial celluloseaerogel.

The method can further include the step of forming the bacterialcellulose aerogel from a bacterial cellulose hydrogel.

The bacterial hydrogel can be synthesized bacterial cellulose cultivatedusing Kombucha strains.

The carbon nanofiber network can include cellulose nanofibers extractedfrom plants or biomass.

The carbon nanofiber network can include synthesized polymer nanofibers.

The synthesized polymer nanofibers can be one or different nanofibers.

The polymer nanofiber can be made from phenolic resin.

The polymer nanofiber can be made from polyimide or polyacrylonitrile.

The step of employing the plasma can occur for at least 5 minutes.

The step of employing the plasma can occur for at least 15 minutes.

In general, in another embodiment, the invention features a method ofmaking an AC-supercapacitor. The method includes selecting a materialcomprising cellulose paper. The method further includes placing thecellulose paper within a plasma chamber. The method further includesflowing a gas comprising carbon feedstock gas into the plasma chamber.The method further includes employing a plasma within the plasma chamberto carbonize the cellulose and to deposit edge oriented graphene on thematerial to create an edge oriented graphene (EOG) electrode material.The method further includes forming a EOG electrode from the EOGelectrode material. The method further includes incorporating the EOGelectrode in a cell to form an AC-supercapacitor. The supercapacitor isoperable for running at frequencies of at least 0.1 kHz.

Implementations of the invention can include one or more of thefollowing features:

The AC-supercapacitor can be operable for running at frequencies of atleast 1 kHz.

The step of employing the plasma can include utilizing a microwave or RFor DC plasma enhanced chemical vapor deposition system.

During the step of employing the plasma, flowing the gas in the plasmachamber can be at a pressure of less than 50 Torr.

The carbon feedstock gas can include methane or acetylene.

The gas can further include a non-carbon based gas.

The gas can further include H₂ or ammonia.

The step of employing the plasma can occur for at least 5 minutes.

The step of employing the plasma can occur for at least 15 minutes.

The EOG layer can include multiple atomic graphene layers.

The step of incorporating the EOG electrode in the cell to form theAC-supercapacitor can include stacking a plurality of EOG electrodes inthe cell.

In general, in another embodiment, the invention features anAC-supercapacitor. The AC-supercapacitor includes a pair of carbonnanofiber network (CNN) electrodes, a separator to separate theelectrodes, and electrolytes between the electrodes. TheAC-supercapacitor is operable for running at frequencies of at least 0.1kHz.

Implementations of the invention can include one or more of thefollowing features:

The AC-supercapacitor can be operable for running at frequencies of atleast 1 kHz.

The AC-supercapacitor can use aqueous electrolyte.

The AC-supercapacitor can use organic electrolyte.

The AC-supercapacitor can have a voltage operation window of at leastabout 2 V.

The CNN electrode can include carbonized bacterial cellulose aerogel.

The electrolyte can be an aqueous electrolyte.

The electrolyte can be an aqueous organic electrolyte.

In general, in another embodiment, the invention features anAC-supercapacitor. The AC-supercapacitor includes a pair of edgeoriented graphene (EOG) electrodes, a separator to separate theelectrodes, and electrolytes between the electrodes. TheAC-supercapacitor is operable for running at frequencies of at least 0.1kHz.

Implementations of the invention can include one or more of thefollowing features:

The AC-supercapacitor can be operable for running at frequencies of atleast 1 kHz.

The AC-supercapacitor can have a voltage operation window of at leastabout 2 V.

The EOG electrode can include carbonized cellulous paper.

The electrolyte can be an aqueous electrolyte.

The electrolyte can be an organic electrolyte.

The AC-supercapacitor can include a stack of EOG electrodes.

In general, in another embodiment, the invention features a device thatincludes one of the above above-described AC-supercapacitors.

Implementations of the invention can include one or more of thefollowing features:

49. The device of claim 48, wherein the device is selected from a groupconsisting of consumer devices, medical devices, and defense devices.

The device can be selected from a group consisting of devices utilizedin telecomm, industry, renewable energy, and power transmissioninfrastructures.

The can be selected from a group consisting of devices utilized inlegacy electric systems for filtering, bypass, decoupling, and burstpower functions.

The device can be a renewable energy device.

In general, in another embodiment, the invention features a method. Themethod includes utilizing an AC-supercapacitor running at a frequency ofat least 0.1 kHz. The AC-supercapacitor includes a carbon nanofibernetwork (CNN) electrode.

Implementations of the invention can include one or more of thefollowing features:

The method can include utilizing the AC-supercapacitor running at afrequency of at least 1 kHz.

The AC-supercapacitor can be utilized for ripple current filtering,power conditioning, energy storage, or pulse generation.

In general, in another embodiment, the invention features a method. Themethod includes utilizing an AC-supercapacitor running at a frequency ofat least 0.1 kHz. The AC-supercapacitor includes an edge orientedgraphene (EOG) electrode.

Implementations of the invention can include one or more of thefollowing features:

The method can include utilizing the AC-supercapacitor running at afrequency of at least 1 kHz.

The AC-supercapacitor can be utilized for ripple current filtering,power conditioning, energy storage, or pulse generator.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention will be apparent from the following description of embodimentsas illustrated in the accompanying drawings, in which referencecharacters refer to the same parts throughout the various views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating principles of the present invention:

FIG. 1 is an illustration depicting a processes of the present inventionto make an edge oriented graphene (EOG) electrode material.

FIG. 2A is a photo image of cellulose paper and an EOG electrodematerial.

FIG. 2B is a graph of the XPS spectra of the cellulose paper and the EOGelectrode material (labelled as EOG/CCP) to show the C/O compositionratio change.

FIG. 2C is a graph of the XRD patterns of the cellulose paper and theEOG electrode material.

FIGS. 3A-3C are scanning electron microscope (SEM) images andtransmission electron microscope (TEM) images of an edge orientedgraphene (EOG) electrode material.

FIG. 4. shows an AC-supercap packaged into coin cell form that was madeusing electrodes of the present invention.

FIG. 5A is a graph of a Nyquist impedance plot of the electrochemicalimpedance spectroscopy (EIS) characterization of an AC-supercap cellmade from EOG electrodes.

FIG. 5B is a graph of a Bode plot of an AC-supercap cell having EOGelectrodes.

FIG. 5C is a graph showing the calculated areal and volumetriccapacitances of the EOG electrode at different frequency.

FIG. 5D is a graph showing the dependence of real (C′) and imaginarypart (C″) of the electrode areal capacitance on frequency.

FIG. 6A is a graph showing the capacitance of the EOG electrodematerials at different frequencies for 1, 2, and 3 layered EOG electrodematerial sheets.

FIG. 6B is a graph showing the dependence of areal capacitance on layernumber (1, 2, and 3 layers) at three frequencies to show the sub-linearincrease of capacitance with EOG electrode material layers.

FIG. 6C is a C″(ω) plot for the 1, 2, and 3, layered EOG electrodematerial sheets.

FIG. 7 is an illustration depicting a processes of the present inventionto make a carbon nanofiber network (or nanoweb) (CNN) electrodematerial.

FIG. 8A is a photo of bacterial cellulose (BC) pellicle cultivated incontainer.

FIG. 8B is a photo of a piece of BC hydrogel made from the BC pellicleshown in FIG. 8A.

FIG. 8C is a BC aerogel made from the BC hydrogel shown in FIG. 8B.

FIG. 8D is a photo of carbon nanofiber network (CNN) electrode materialmade from the BC aerogel shown in FIG. 8C.

FIGS. 9A-9C are scanning electron microscope (SEM) images andtransmission electron microscope (TEM) images of the BC aerogel shown inFIG. 8C.

FIG. 9D is a graph of an XRD pattern of the BC aerogel shown in FIG. 8C.

FIGS. 10A-10B are scanning electron microscope (SEM) and transmissionelectron microscope (TEM) images of a carbon nanofiber network (CNN)electrode material.

FIGS. 11A-11B are graphs of EIS spectra and capacitance of CNN electrode(thickness 20 μm) based aqueous cells. FIG. 11A is a Nyquist plot of thecell spectra, with the high-frequency region shown in the inset 1101.FIG. 11B is a graph showing dependence of cell phase angle and electrodecapacitance on frequency.

FIGS. 11C-11D are graphs showing a comparison of the three CNNelectrodes with different thickness in the aqueous electrolyte. FIG. 11Cshows cell phase angle vs. frequency. FIG. 11D shows areal capacitancevs. frequency.

FIG. 11E is a graph that shows capacitance cycling stability tested fora CNN electrode (thickness 10 μm) based aqueous cell at 50 mA cm⁻², withthe inset 1109 as a section of the C-D curve.

FIGS. 12A-12B are graphs of EIS spectra and capacitance of CNN electrode(thickness 10 μm) based organic electrolyte cells. FIG. 12A is a Nyquistplot of the cell spectra, with the high-frequency region shown in theinset 1201. FIG. 12B is the Bode plot of the spectrum.

FIG. 12C is a graph showing areal capacitance vs. frequency for thethree CNN electrodes in the organic electrolyte.

FIG. 12D is a graph of cyclic voltammetry (CV) of one CNN electrode(thickness 10 μm) based organic cell up to a rate of 1000 V s⁻¹ in apotential window 0-2.5 V.

FIG. 12 E is a graph of CV scans at 100 V s⁻¹ in different potentialwindows up to 3.5 V.

FIG. 12F is a graph of the cell stability test up to 100K cycles withthe inset 1209 as a section of C-D curve.

FIGS. 13A-13B are, respectively, a schematic and photo of a circuit usedfor pulse energy harvesting and storage to demonstrate of capability ofAC-supercaps for environmental pulse energy harvesting. The inset photo1309 in FIG. 13B is showing that the charged AC-supercap can turn on agreen LED (the LED running time depends on the stored energy).

FIG. 13C is a graph of the generated electric pulses from finger tappingof the piezoelectric element shown in the circuit of FIGS. 13A-13B, andthe pulse energy that was stored into the AC-supercap shown in FIGS.13A-13B. The irregular pulse voltage generated by the piezoelectricelement under hand finger tapping (V_(i)) and the voltage across theAC-supercap (V_(o)) when powering a mega ohm resistive load.

FIG. 14A is a schematic of a circuit used to demonstrate capability ofAC-supercap for ripple current filtering.

FIG. 14B is a graph showing the input voltage wave applied into thecircuit of FIG. 14A, and the rectified pulse wave when the AC-supercapwas not connected in the circuit of FIG. 14A, and the output DC voltageafter AC-supercap filtering in the circuit of FIG. 14A. A resistive loadwas applied for the two outputs.

DETAILED DESCRIPTION OF THE INVENTION

The present invention are ultra-high rate (ultra-fast, high-frequency)supercapacitors that can respond at hundreds hertz or kilohertzfrequencies (AC-supercapacitors). With a compact size, AC-supercaps canbe used in lieu of ubiquitous (but bulky, high loss and low-performance)aluminum electrolytic capacitors (AECs), for line-frequency ripplecurrent filtering, power conditioning, and many other functions neededin any electric power system design and electronic circuit design. Thisis a disruptive technology that has huge impacts on compact andefficient power supplies and power electronics. They are also suitablefor pulse energy storage harvested form environment, for pulse powergeneration to drive actuators, for strobes or burst communication, orshort pulse generation for wearable or implantable devices to provideaid or support the smooth functions of the muscle, organs, or heart suchas cardiac pacemaker.

The electrode material and its nanostructure of the present inventionprovide the boost of the response frequency of the supercapacitors fromquasi-DC (<a few Hz) to above 1 kHz AC. Due to the contradictionrequirements of high frequency and high capacitance density, kHzAC-supercaps with much compact size and better performance over AECshave been constructed, including those with cutoff frequencies of morethan 3 kHz and an areal capacitance of more than 3 mF/cm².

The electrodes utilized in the present invention can be formed by usingedge orientated multilayer graphene that are grown around carbonizedcellulous paper where the carbonization of the cellulous paper andgraphene deposition are implemented in one step. The electrodes of thepresent invention can also be formed by using an interconnected carbonnanofiber based carbon nanofiber network (CNN).

Edge Oriented Graphene (EOG) Electrode Material

Flexible, thin, light, and freestanding edge oriented graphene (EOG)electrode materials, made from rich and renewable cellulose resource,are excellent and more practical electrodes for AC-supercapacitors.

FIG. 1A illustrates a method for creating edge oriented graphene (EOG)electrode materials. The method includes placing cellulose paper 101(such as Kimwipes® wiper sheets) in a plasma chamber 102 that can createa plasma (such as a microwave plasma enhanced chemical vapor deposition(PECVD) system). Alternatively, a chamber that creates a RF or DC plasmacan be employed. The plasma is represented by dashed circle 103. Theplasma chamber 102 is pumped down to a low pressure (such as 1×10⁻⁴Torr). A gas including a carbon feedstock gas (such as a gas mixture ofmethane) from source(s) 106 is then flowed into the plasma chamber 102.For example, the chamber is refilled with a mixture of methane and H₂gas at a pressure of 30 Torr. The temperature inside the chamber wherethe cellulose paper sits is controlled at a certain temperature (such as750° C.). A certain (such as 1 kW) microwave radiation was applied tocreate plasma. Carbonization/graphitization of the cellulose paper andedge oriented graphene (EOG) is carried out simultaneously, such as in 1kW CH₄/H₂ plasma for 5 minutes (which plasma was estimated to be at atemperature of around 1200° C. or higher).

In this process, the cellulose decomposed or reduced due to activehydrogen species in the plasma and converted into amorphous carbon andfurther partially graphitized due to high temperature, resulting incarbonized cellulose sheet (carbon fiber sheet with each fiber having adiameter such as several micrometers). Meanwhile, active carbon speciesfrom carbon feedstock plasma, assisted with hydrogen species, willdeposit on the carbonized cellulose sheet forming perpendicularlyoriented graphene flakes circling around each carbon fiber in the sheet,with graphene edges fully exposed, the so-called edge-oriented graphenenetwork structure, or EOG. This resulted in EOG coated carbon fibersheets 104 (also referred to herein as the EOG electrode materials).

With massive loss of oxygen and hydrogen and part of carbon compositionsduring plasma pyrolysis, the cellulose paper 101 and the EOG electrodematerial 104 exhibit very different morphology and composition. Such EOGelectrode materials have a nominal area reduced by a factor of 5-6 (FIG.2A) and mass density was changed from 2.1 mg cm⁻² to 1.1 mg cm⁻², ascompared to the cellulose paper 101. The contraction of dimensions isanisotropic due to the cellulose texture of the cellulose paper 101. Asshown in FIG. 2B, X-ray photoelectron spectroscopy (XPS) measurement wasconducted to compare the composition change. For cellulose paper 101(plot 201), the atomic ratio of oxygen over carbon is about 3:2, whileit became 1:24 for EOG electrode material 104 (plot 202). The remainingtrace amount of oxygen composition mainly exist in the C—O bonding form.The material structure was characterized by X-ray diffraction (XRD) andRaman spectroscopy. As shown in FIG. 2C, the cellulose paper 101 (plot203) displays the characteristic XRD peaks of cellulose at 15.4°, 16.2°,22.5°, and 34.5°. After pyrolysis and growth, the EOG electrode material104 (plot 204) shows peaks at ˜26.2°, corresponding to thecharacteristic (002) peak of graphite, indicating that the cellulosefibers were graphitized in the high-temperature EOG growth process.

In the Raman spectrum of EOG electrode material 104, characteristic D,G, 2D, and 2D′ peaks of graphene/thin-graphite are clearly identified.The intensity ratio of 2D peak and G peak, I_(2D)/I_(G), is ˜1.31,suggesting that each graphene flake consists of multiple graphene atomiclayer.

FIG. 3A (EOG coated carbon fiber sheet) and FIG. 3B (zoom-in on EOG) area scanning electron microscope (SEM) images of an EOG electrode materialmade by the process shown in FIG. 1. Inset 301 (of FIG. 3A) and FIG. 3Care a transmission electron microscope (TEM) images of the EOG electrodematerial made by the first process. Arrows 302 point to the distributedgraphene edges in the EOG electrode material.

Each individual carbon fiber has a size of a few micrometer in diameter,in contrast to the tens micrometer size of the pristine cellulose fiber.A single sheet of EOG electrode material is approximately 10 μm thickbased on cross-sectional view of imaging. Most carbon fibers have asolid core, but some of them do have a hollow feature. The grapheneflake network, with a maze-like morphology, has a height of severalhundred nanometer and fully encompasses individual carbon fibers. Themorphology of edge-oriented graphene flake network are further revealedby the TEM images.

Densely-packed graphene flakes grown on the surface of carbon fibers aredisplayed (FIG. 3B), while the perpendicularly oriented and fullyexposed knife-like graphene edges are clearly observed in insert 301 ofFIG. 3A. As shown in FIG. 3C, an individual graphene flake consisting ofmultiple atomic layer is shown in FIG. 3C, disclosing its taperedprofile with a wide base and narrow tip, and along the perpendicularflake walls are distributed with dense graphene edge steps (arrows 302).

Such a freestanding network of the EOG electrode material combinesseveral unique features together, very suitable for developingAC-supercaps with high capacitance density. First of all, the edgeoriented graphene network provides high density and easily accessiblegraphene edges. These atomic edges or steps have rich adsorption sitesfor electrolyte ions, and therefore, offer much higher charge storagecapacity than its basal plane. It was reported that the edge plane ofgraphite offers a capacitance of 50-70 μF/cm², around 20 times higherthan its basal plane. The edge orientation of graphene flakes, withsub-μm flake height and wide opening channels between flakes, minimizesthe porosity effect which otherwise would restrict ionic transport, andhence the high frequency response could be achieved. The well-connectedEOG electrode material delivers a high electronic conductivity tominimize the series resistance.

Unlike previously reported VOG grown on a 2-D planar substrate with alimited surface area, EOG growth around the porous 3-D carbon fibernetwork offers a large surface area for a given footprint, particularlythe micrometer-size macropores (FIG. 3A) allow the EOG electrodematerials to stack together without sacrificing frequency response.Therefore, a large areal capacitance, as well as a large volumetriccapacitance and a large mass-specific capacitance is achieved by furtherconsidering their small volume and low mass.

FIG. 4 shows a AC-supercap, packed into coin cell form 401 that can bemade that includes two same electrodes 406, which can be made from theEOG electrode material 104. Such coin cell 401 further includes a metalcap 402, a spring 403, a spacer 404, a separator 405, the electrodes406, and a metal case 407. Not shown are the electrolyte which is soakedin the porous separator and the two electrodes with macro-pores.

For example, coin cells 401 were made utilizing the EOG electrodematerial 104 made as described above, as follows: Using a single sheetEOG electrode material 104 with a thickness of 10 μm as the electrodes406 (cut to the proper size), symmetric cells (e.g., two electrodes arethe same) were assembled to test their performance. The electrodes 406(the EOG electrodes) were soaked in an aqueous electrolyte (6 M KOHaqueous electrolyte) for 12 hours and then assembled in coin cells 401with a 25 μm thick separator.

The stability of the aqueous electrolyte was found to limit the workingvoltage to less than ˜0.9 V.

The Nyquist plot of the measured impedance Z(ω) is shown in FIG. 5A,with an expanded view of high frequency part in the inset 501. The plotdisplays a nearly vertical straight line at a frequency below 29 kHz,behavior of a near-ideal capacitor connected with an equivalent seriesresistance (ESR). At high frequency, it has a small Warburg 45° lineregion and a tiny curvature. A Warburg line region and a largesemicircle are common features of porous electrodes. Here due to thewide opening between perpendicular graphene flakes, the microporouseffect, which limits the frequency response, is significantly inhibited.Only low density of pores exists in the EOG electrode materialstructure, due to either hollow fibers or expanded spacing betweenatomic graphene layers, which contributes the tiny Warburg line andcurvature region at frequency above 29 kHz. ESR, indicated by theintersection of impedance curve with the Z′ real axis is 79 mΩ for sucha cell with an electrode nominal area of 0.5 cm², which is similar to orsmaller than that of other ultrafast capacitors based on VOG, graphenefoam and graphene hydrogel. ESR of a capacitor will ultimately determineits frequency response when considering the effect of RC constant, and asmall ESR is critical for high frequency response AC-supercaps. Such alow ESR suggests a highly conductive network offered by the EOGelectrode materials for electrons while its porous structurefacilitating the electrolyte ion transport.

The corresponding impedance phase angle data vs. frequency (Bode plot)is shown in FIG. 5B. An ideal capacitor, with its impedance of 1/jωC,exhibits 90° phase angle, while in a real supercapacitor, due to theresistive component (R), the phase angle will be lower, particularly athigh frequencies. Therefore, the frequency ƒ₀, corresponding to 45°phase angle of the impedance (R+1/jωC), delineates the frequencyboundary, below which the device behaves more like a capacitor, whileabove which, it manifests more like a resistor. Therefore, ƒ₀ can bedefined as the characteristic cutoff frequency of capacitors. Of course,the capacitor should work at a frequency below ƒ₀ with a large absolutevalue of phase angle (˜>80°) in minimizing resistive energy loss. It isnoted that the AC-supercap cell having EOG electrodes shows an absolutevalue phase angle above 80° up to a frequency of ˜1 kHz, and acharacteristic cutoff frequency of ˜10 kHz. Outside of the Warburgregion in Nyquist plot of its impedance, the cell can be simplified as aresistance and a constant phase element (CPE) in series. If the CPEexponent is assumed to be 1, this becomes the commonly used series RCmodel as an approximation. Then the frequency-dependent arealcapacitance of the electrode can be derived as:

C _(A)=(−1/ωZ″)×(2/A)=−2/ωZ″A   (1)

where A is the footprint area of EOG electrode. The derived areal andvolumetric capacitances per electrode are plotted in FIG. 5C (plots 502and 503, respectively, based on series RC model using Eq. 1). At 120 Hz,these two numbers are 0.6 mF cm⁻² and 0.6 F cm⁻³ for the EOG electrodes.The corresponding phase angle at 120 Hz is −83°, comparable with thestate of the art such as VOG on Ni substrate (−82° to −85°), POG in Nifoam (−82°), electro-chemically reduced graphene oxide (ErGO) on Au(−84°), and patterned graphene CNT on metal coated silicon (−81.5°), butwith a much larger volumetric capacitance when the whole electrodevolume is considered. Not shown here, the areal and volumetriccapacitances have been further increased to more than 1 mF cm⁻² and 1 Fcm⁻³ for the EOG electrodes after optimizing the deposition process.

The frequency-dependent capacitance can also be estimated based on themethod of Taberna et al., “Electrochemical Characteristics and ImpedanceSpectroscopy Studies of Carbon-Carbon Supercapacitors” J. Electrochem.Soc. 2003 150, A292-A300 to define complex capacitance C_(C)(ω) as:

C _(C)(ω)=C′(ω)−(jC″(ω))=1/jωZ   (2)

where C′(ω) is the accessible capacitance at the correspondingfrequency, while C″(ω) corresponds to energy dissipation due toirreversible process (resistive loss). The evolution of C″(ω) withfrequency reaches a maximum at the cutoff frequency ƒ₀ that defines therelaxation time constant τ₀. The electrode areal capacitance, calculatedas C_(A)=2C′/A, and C0″ are plotted in FIG. 5D (plots 504 and 505,respectively, obtained from Eq. 2). Again, an electrode capacitance of˜0.6 mF cm⁻² is obtained at 120 Hz, which is consistent with the numberextracted from the series-RC model in Eq. (1). Furthermore, the plotshows that C″(ω) reaches a maximum at the cutoff frequency ˜10 kHz,which gives a relaxation time constant of ˜0.1 ms. This value is orderslower than most reports on high-rate supercapacitors, such as onion-likenanocarbon (26 ms), graphene film (13.3 ms), graphene-conducting polymer(1.0-2.9 ms), comparable or better than the state-of-the-art ofhigh-frequency supercapacitors based on ErGO/Au (0.17-1 ms), POG in Nifoam (0.248 ms), or VOG on Ni substrate (0.067 ms).

One motivation to develop AC-supercap is to replace the bulky AEC bycompact AC-supercap for AC line-frequency current ripple filtering orother high-frequency applications. Therefore, these AC-supercaps musthave a compact size with a large volumetric capacitance in term of thewhole package. Therefore, the active material should have enough loadingin the electrode when considering the inactive volume in a packed cell.Most reports in the literature, based on sub-μm thick active layers,significantly fall short in this aspect. It is obvious that for a thickactive layer to response in the kHz range, macropores are necessary forfree electrolyte ions transporting deeply into the thick layer. Themacropores in the hierarchical EOG electrode material might afford thiscapability even when several sheets are stacked together for a thickerelectrode that is necessary for practical devices with a largecapacitance. Therefore, the cell performance of electrodes based on 1-,2-, and 3-layered sheets of EOG electrode material were tested andcompared.

In FIG. 6A, frequency-dependent areal capacitances (plots 601-603 for1-, 2-, and 3-layers, respectively), based on EIS measurement andfurther derived from Eq. (2) are compared, and particularly the arealcapacitances at 10, 120, 1000 Hz are plotted in FIG. 6B (plots 604-606,respectively). From 1-layer, 2-layer, to 3-layer, the 120 Hz arealcapacitance increases from 0.6 mF cm⁻² to 1.1 and then 1.5 mF cm⁻², and1 kHz capacitance is 0.5, 1.0, and 1.4 mF cm⁻², respectively, nearlyproportional to the number of layers, and therefore the volumetricdensity only has slight degradation as the layer number increases. Thesame is also true for other frequencies.

The cutoff frequency and relaxation time constant, as indicated by FIG.6C (plots 607-609 for 1-, 2-, and 3-layers, respectively), only haveslight degradation as electrode thickness increases. Particularly, thecutoff frequency changes from ˜10 kHz to ˜5 kHz, when the EOG electrodeincreases from 1-layer to 2-layer, while there is no further obviouschange from 2-layer to 3- layer. This nonlinear change of cutofffrequency is believed to be related to the cell assembling process. Whenseveral layers are stacked to form one electrode, the mechanical forceapplied may have significant impact on the contact resistance betweenlayers. However, as far as a good contact is established between thesestacked layers, the EOG electrode based cells, even with a largeelectrode thickness, are capable to achieve high frequency response witha cutoff frequency above kHz range. In other embodiments, the startingcellulose paper can be much thicker than utilized in the embodimentstested above. In such case one layer would be thick enough as electrodeand there would be no layer-to-layer contact issue.

Carbon Nanofiber Network (CNN) Electrode

Through a rapid plasma pyrolysis process, a carbon nanofiber network(CNN) electrode material, which consists of interconnected carbonnanofibers (such as a carbonized bacterial cellulose (CBC) aerogel) issuitable as electrode for kHz AC-supercap with extremely large arealcapacitance (at 120 Hz). The voltage operation window can be increasedto ˜3 V when an organic electrolyte used (instead of an aqueouselectrolyte). Their excellent performance is attributed to the suitablepore size and well-crosslinked nanofiber structure of CNN electrodematerial (such as CBC aerogel) that can simultaneously offer a highionic conductivity for the electrolyte in the porous electrode, a highelectronic conductivity and a large surface area, resulting in highfrequency response and large areal capacitance. The potential of theseAC-supercaps for current ripple filtering in AC/DC converter andhigh-frequency pulse energy storage has been demonstrated, showing thatthese AC-supercaps can be utilized in these and other similarapplications.

FIG. 7 illustrates a method for creating carbon nanofiber network (CNN)electrode materials. The method includes placing cellulose nanofibernetwork 701 (such as produced by a microbial fermentation process) in aplasma chamber 102 that can create a plasma (such as a microwave plasmasystem). The plasma is represented by dashed circle 103.

The cellulose nanofiber network produced by microbial fermentation canbe bacterial cellulose (BC) cultivated using Kombucha strains (themethod used for production of Kombucha tea), as described in Zhu, C. etal., “Kombucha-synthesized bacterial cellulose: Preparation,characterization, and biocompatibility evaluation,” Journal ofBiomedical Materials Research Part A 2014, 102 (5), 1548-1557.

For example, black tea (5 g) was boiled in 500 mL water for 5 min toobtain tea solution. After cooling down, 100 g sucrose, 5 g yeastextract, and 0.5 g peptone were added to the tea. By adding water andacetic acid, 1 L culture solution was made with a pH value of 5.0. Usinga shallow glass container, the Kombucha strains were cultured in thesolution for several days or weeks, depending on the BC pelliclethickness required. The cultivation temperature was maintained at ˜30°C. The obtained new pellicles were washed with water to remove theculture solution and then boiled in 1.0% NaOH solution for 2 h toeliminate the bacterial cells. After washing in de-ionized water, BChydrogels were obtained.

FIG. 8A is a photo of the BC pellicle cultivated in a container. FIG. 8Bis a photo of a small piece of BC hydrogel after cleaning made from thisBC pellicle.

The BC aerogel is then formed from the BC hydrogel. For example, thiscan be done by freeze drying the BC hydrogel dried at −80° C. overnightfollowed by vacuum drying to remove all water content to obtain BCaerogels such as a tiny piece shown in FIG. 8C. The BC aerogel has amass of less than 1% of its hydrogel counterpart. FIG. 9A is an SEMimage of the BC aerogel shown in FIG. 8C. FIGS. 9B-9C are TEM images ofthe BC aerogel shown in FIG. 8C. The SEM of FIG. 9A image reveals thethree-dimensional web structure of BC aerogel, while the crosslinkbetween nanofibers or branch structure is noted in the TEM image (FIG.9B). While other natural celluloses can be used in embodiments of theinvention, the BC aerogel is performed over other natural celluloses dueto smaller fiber size (around 10-50 nm) and higher degree ofcrystallinity and purity.

The x-ray diffraction (XRD) pattern (FIG. 9D) indicates goodcrystallization of the cellulose. According to the cellulose I_(α) aindexation, the three peaks corresponding to 2θ diffraction angles of14.45, 16.75 and 22.75° are indexed to (100), (010) and (110)crystallographic planes. The high-resolution TEM image (FIG. 9C) showsthe (110) lattice planes with a plane spacing of 0.38 nm. Thecrosslinked gel structure of BC is expected to yield excellentintra-electrode conductivity after pyrolysis. These nanoscale fibersalso offer larger surface area compared to other cellulose materialscomposed of microscale large fibers.

Pyrolysis of synthesized polymer and cellulose polymer precursors,including BC, to derive carbon material has been commonly conducted in athermal process with N₂ or Ar environment at temperature more than 800°C. for a few to more than 10 hours. Micropore activation during thislong-duration thermal process, through physical decomposition andchemical etching is also a very common practice for achieving a largesurface area for electrode applications. However, this energy-consumingthermal pyrolysis process to introduce micropores, is detrimental forthe electrode frequency response, and therefore is not suitable forAC-supercaps.

Referring back to FIG. 7, the cellulose nanofiber network 701 (such asthe BC aerogel shown in FIG. 8C) is placed in plasma chamber 102 and anRF or microwave plasma is employed, such as a microwave plasma using amicrowave plasma chemical vapor deposition system. The plasma chamber102 is pumped down to a low pressure (such as 1×10⁻⁴ Torr). However,different than the processes illustrated in FIG. 1 for the EOG electrodematerial, the gas that is used to flowed into the plasma chamber 102from source(s) 706 are is not used as carbon feedstock gas (i.e., nocarbon deposition from the carbon feedstock gas). Nonetheless, gaseswith carbon (such as methane) can be, and are, utilized in embodimentsof the present invention. To avoid carbon deposition from thehydrocarbon plasma source that could block the pores in the carbonizedBC aerogel and result in low frequency response, the BC aerogel had nodirect exposure to the plasma through a graphite screening sheet.

For example, the chamber is refilled with a mixture of methane (50 sccm)and H₂ (100 sccm) at a pressure of 30 Torr. The temperature inside thechamber is controlled at 750° C. 1 kW microwave radiation was applied tocreate plasma. It was observed that a short period (15 minutes) ofplasma pyrolysis of the cellulose nanofiber network 701 produced carbonnanofiber network (CNN) electrode material 704 with excellent electronicconductivity, large interconnected pores for rapid electrolyte ionmigration, and reasonable surface area. Without employing any furthertreatment, such as structural modification, doping of electrode materialor applying metallic current collector that were commonly used in otherstudies, the obtained carbon nanofiber network (CNN) electrode material704 was demonstrated to be very suitable for AC-supercaps. FIG. 8D is aphoto of the carbon nanofiber network (CNN) electrode material 704formed by this process.

After plasma pyrolysis, the carbon nanofiber network (CNN) electrodematerial 704 area was shrunk to about 30% and the mass was largelyreduced. The morphology of the CNN electrode material 704 is shown inthe SEM image of FIG. 10A. The overall structure is highly porousnetwork with meso- and macro-pores formed by the well-connected carbonnanofibers (CNF). The textural properties of CBC membrane were furthercharacterized by N₂ sorption measurements. A Brunauer-Emmett-Teller(BET) surface area of 57.5 m² g⁻¹ was measured, with a pore volume of0.374 cm³ g⁻¹. The pore size distribution showed a minimum pore diameterof 3.8 nm and the CNN electrode material was dominated by meso- andmacro-pores. Without micropores, slit pores, and dead end pores, theinterconnected large pores allows rapid transportation of ions inelectrolyte through the electrode mesh with a low ionic resistance.Unlike a cluster simply stacked by individual CNFs or CNTs that has nointimate mechanical and electrical connection between individual fibers,well-connected CNN electrode material gel is formed by the branchstructure of fibers with intrinsic connection. Such a unique branchedweb-like CNN electrode can simultaneously offer a high ionicconductivity and a high electronic conductivity, essential for highfrequency response in a 3D distributed electrode. The derived CNNelectrode material is amorphous carbon (FIG. 10B) without showing XRDpeaks, and some of them have a tubular-like or sheath structure.

Such CNN electrode material can also be incorporated into a coin cell,such as shown in FIG. 4.

Aqueous Electrolyte Cells

CNN electrodes were studied with three different thicknesses of 10, 20and 60 μm, named as CBC-10, CBC-20 and CBC-60, respectively. (“CBC”stands for carbonized BC) The performance of these electrodes wasevaluated in 6M KOH aqueous electrolytes. The frequency dependentbehaviors were characterized via electrochemical impedance spectroscopy(EIS) measured form 100 kHz to 1 Hz, with a sinusoidal AC voltage of 10mV amplitude, using the assembled symmetric coin cells.

The Nyquist plot of EIS for CBC-20 based aqueous cells is presented inFIG. 11A, with the zoom-in view of the high frequency region shown asthe inset 1101. A knee frequency of ˜4.5 kHz is noted. Below thisfrequency, the spectrum is approximately a straight line with a slopeapproaching 90°, and therefore the cell can be approximated by RC inseries. Above the knee frequency, a transition region and a semicirclefeature might be identified, which is ascribed to the distributedcapacitance and resistance of the porous CNN electrode, a possible minorinsulating layer between the coin cell package and the free-standing CNNelectrode that has no extra current collector, and possibly the minorcharge transfer at the electrolyte and electrode interface due to smallfraction of oxygen-related remnants in CBC. These features result in asmall equivalent series resistance (ESR) of 9 mΩ·cm² in conjunction withan equivalent distributed resistance (EDR) of 26 mΩ·cm².

The Bode plot of the same EIS spectrum is presented in FIG. 11B (plot1102), indicating the absolute value of the phase angle generally isabove 80° for frequencies up to a few hundred Hz. In particular, thecell at 120 Hz show a phase angle of −82°, indicating that it maintainsthe capacitive nature very well at such a frequency. The characteristicfrequency at −45° is around 3.3 kHz.

Using the RC model, the capacitance below the knee frequency wascalculated and the dependence of the areal capacitance (C_(A)) of CBC-20electrode on frequency is also shown in FIG. 11B (plot 1103). Anelectrode areal capacitance as large as 2.98 mF cm⁻² at 120 Hz wasmeasured. The RC time constant at 120 Hz is 0.18 ms.

The electrode thickness effect on capacitance and frequency response arepresented in FIGS. 11C-11D for the three thicknesses of the CNNelectrodes. For FIG. 11C, plots 1103-1105 are for thickness of 10, 20and 60 μm, respectively. For FIG. 11D, plots 1106-1108 are for thicknessof 10, 20 and 60 μm, respectively. For a frequency up to 200 Hz, thesegenerally shows a phase angle between −80° to −90°, and thecharacteristic frequency at −45° phase occurs at 4.1, 3.3 and 1.3 kHzfor CBC-10, CBC-20 and CBC-60, respectively. In particular, the areacapacitance at 120 Hz is 1.51, 2.98 and 4.50 mF cm⁻² for the threeelectrodes. It is noted that for the two thinner electrodes, thevolumetric capacitance maintains a constant (˜1.50 mF cm⁻³) with theareal capacitance linearly increasing. Although CBC-60 gives a very highvalues for areal capacitance, its volumetric counterpart (0.75 mF cm⁻³)falls lower compared to the thinner ones. CBC-20 offers a large arealcapacitance while maintaining the volumetric efficiency.

Cyclic voltammetry (CV) and Galvanostatic charge-discharge (C-D) cyclingwas further studied. Capacitance cycling stability was tested for aCBC-10 aqueous cell (FIG. 11E). After 100,000 continuous cycles withfull charge and discharge, the capacity was maintained at a valueapproximately 95% of its initial capacity. Very few studies onhigh-frequency supercapacitors reported such long-cycling stability. Theslight degradation of the capacitance might be related to aqueouselectrolyte evaporation due to packaging.

Organic Electrolyte Cells

CBC-10, CBC-20 and CBC-60 three electrodes were also studied in organicelectrolyte (such as 1 M tetraethylammonium tetrafluoroborate (TEABF4)in anhydrous acetonitrile (AN) solution). FIG. 12 A is the complex-planeimpedance spectrum for a CBC-10 based organic cell. A knee frequency of3.0 kHz was found, below which the spectrum can be approximated by a RCseries model. The sum of ESR and EDR is approximately 0.33Ω·cm². Theplot of phase vs. frequency in FIG. 12B indicates a phase angle of −80°at 120 Hz.

The areal capacitance vs. frequency for three electrodes are compared inFIG. 12C (plots 1203-1205 are for thickness of 10, 20 and 60 μm,respectively), with a 120 Hz capacitance of 0.51, 1.08 and 2.55 mF cm⁻²for CBC-10, CBC-20 and CBC-60 electrodes, respectively, and thecorresponding volumetric capacitances of 0.51, 0.54 and 0.425 F cm⁻³.The −45° characteristic frequency occurs at 1.8, 0.55 and 0.11 kHz, forCBC-10, CBC-20 and CBC-60, respectively, and the phase angles at 120 Hzare −80°, −63° and −44°. The ionic conductivity of electrolyte solutionplays a significant role in determining electrolyte dependent frequencyresponse behavior. The ionic conductivity of 6 M KOH solution at roomtemperature is as high as 620 mS cm⁻¹, whereas the conductivity ofTEABF₄ in acetonitrile solution is merely less than a hundred mS cm⁻¹.The relative bulkiness of the ions in organic solution slows the motionof the ions in the electrolyte and especially within the electrodematrix. As a result, organic cells exhibit slower response than theiraqueous counterparts.

In comparison to aqueous cells, organic cells can work at much highervoltage. CV study of a CBC-10 based organic cell was performed up to1000 V s⁻¹ rates in a potential window of 0-2.5 V (FIG. 12D). The CVprofiles 1206-1208 (for 100, 500, and 1000 V s⁻¹, respectively,maintained the desired quasi-rectangular shape even at such high rates.In fact, in TEABF₄/AN organic electrolyte, the electrode may even worksat a potential window up to 3.5 V (FIG. 12E). This is a prominentcharacteristic of the organic electrolyte cells of the present inventionfor high voltage AC-supercap development. As demonstrated in FIG. 12F,an organic cell was tested in 3 V window continuously for 100,000 fullcharge-discharge cycles. It is interesting to notice that except forinitial cycles, the cell capacitance gradually increased during cycling,and it stabilized after about 50,000 cycles, with more than 20% increaseof the capacitance. An increase in capacitance is expected due tocomplete surface area utilization. Surface wetting of nanostructuredelectrode depends on viscosity and surface tension of the electrolyte.Organic electrolyte has high viscosity and lower surface tension, so ittypically takes longer for organic electrolyte to wet the whole activesurface area, resulting the observed capacitance increase.

It was observed that the organic cells exhibit smaller capacitance thanaqueous cells. The relative dielectric constant of the correspondingsolvent medium certainly has a role in determining the double layercapacitance, since water has a dielectric constant of 80.1, more thandouble over that of acetonitrile (37.5).

The difference in ionic radii of solvated ions might also impact thecapacitance. The solvated ionic radii of K⁺ and OH⁻ are 3.31 Å and 3.00Å, respectively. In contrast, the solvated ionic radii of TEA^(+ and BF)₄ ⁻ in acetonitrile are 13.04 Å and 11.56 Å, respectively. For largerions, the number of adhered ions per unit surface area will be smaller.Surface wettability (or the accessible surface area) is perhaps the mostcritical factor. For PECVD grown VOG electrodes, a pure carbon surfacerenders their hydrophobic property, and therefore a lower capacitance inaqueous electrolyte than in organic electrolyte. However, carbonizedcellulose always have oxygen remains and XPS study of our CBC materialsindicated ˜5% oxygen remaining, which renders the CBC of the presentinvention with high water absorption capability to form CBC hydrogel.These three factors result in a larger CBC electrode capacitance inaqueous electrolyte than in organic electrolyte.

Plasma Pyrolysis vs. Thermal Pyrolysis

It is noted that several studies have employed pyrolyzed bacterialcellulose as electrodes for conventional EC, where typical furnace basedpyrolysis and chemical activation technique was adopted. In thosereports, high-density micropores and mesopores were introduced forenhanced energy density. The large electrochemical resistance arisingfrom strong porous effect results in their sluggish frequency response,and therefore, they cannot be applied for AC-supercaps. In embodimentsof the present invention, when the same BC aerogel was pyrolyzed usingthe conventional furnace process at 800° C. for 2 hours in Arenvironment, even without chemical activation, the resulting CNF filmelectrodes exhibited sluggish response with ƒ₀ of 8 Hz in KOH and 3 Hzin TEABF₄/AN organic electrolyte. The CBC electrodes from thelong-duration thermal pyrolysis process have a very slow frequencyresponse and are not suitable for AC-supercaps.

Plasma gases also play a crucial role in determining the CBC propertyand its frequency response. Pure H₂ plasma and CH₄/H₂ (1:2) mixtureplasma was both used for BC pyrolysis, and the obtained CBC hadconsiderable different morphology and frequency response. Even with thesame input plasma power, the sample in pure H₂ plasma had a considerablelower temperature than that in CH₄/H₂ mixture plasma and the etchingeffect by the pure H₂ plasma might also contribute to microporegeneration in CBC. Therefore, CBC pyrolyzed in H₂ plasma had acharacteristic frequency ƒ₀˜100 Hz, more than one order of magnitudelower than in CH₄/H₂ mixture plasma.

It should be emphasized that the nanoscale fibers of BC with a largesurface area is an important factor for achieving large capacitancedensity. Microscale plant cellulose, i.e., conventional Kimwipe tissuepaper, was compared with nanoscale BC. For a same thickness, the formerhad a capacitance ˜5 times smaller than the latter when both wereprepared under the same plasma pyrolysis condition. All these resultsindicated the superiority of plasma pyrolysis of nanoscale BCcrosslinked fibers for use in AC-supercaps.

Applications

In contrast to conventional supercaps that only run at quasi-DC currentby acting as energy storage, AC-supercaps can respond at high frequency,and therefore act as filtering capacitors. They have much largercapacitance density and smaller equivalent series resistance (ESR) thanAECs. Thus, they offer much better performance for power circuits.

Most of AEC functions could be replaced by the compact and much betterAC-supercaps of the present invention. For compact, low-profile andother space-demanding applications, the low capacitance density andhence bulky size of AECs is the acute pain point, while AC-supercap canperfectly solve the problem. For space undemanding but power ortemperature demanding heavy-duty operations, the relative large thermalgeneration due to large ESR and high failure rate with limited lifetimeof AECs are often complained by customers, but lack of alternativetechnologies to fulfill their needs. Surge, peak or pulse current causedpolarity reverse also shortens the AEC lifetime. AC-supercaps with itslow ESR, low loss and potentially high-temperature rating, can alsosolve these pains of AECs. This is significant in that, with billions ofpieces sold per year, AECs are ubiquitous in consumer, medical, anddefense devices, telecomm, industry, renewable energy and powertransmission infrastructures, and many other legacy electric systems forfiltering, bypass, decoupling, and burst power functions. Thisrepresents a multi-billion sub-segment of the global capacitor industry.Furthermore, compact size or slim/flexible format and efficiency ofAC-supercaps will also find applications for wearable or implantabledevices, used for spike filtering, pulse energy storage, and pulsegeneration.

Such replacement of AC-supercaps for AECs includes but not limited to:

-   -   Miniature Design (e.g., thinner flat panel)    -   Smoother Filtering (e.g., solid-state lighting)    -   Higher Efficiency (less power consumption)    -   Longer lifetime

As verify to the potential capabilities (for EOG electrodes and CNNelectrodes), the CBC-10 based organic AC-supercaps were used for pulseenergy storage and ripple current filtering.

For environmental pulse energy harvest testing, a piezoelectric element(CEB-44D06 from CUI Inc., Tualatin, Oreg.) was used to generate a pulsedvoltage signal from external mechanical noises. The vibration of a motoror hand finger tapping was used to simulate the environmental pulseenergy. The testing circuit is shown in FIGS. 13A-13B. The schematic ofFIG. 13A includes the piezoelectric element 1302 that picks upenvironmental noise and the AC-supercap 1302. The photo of FIG. 13Bincludes a switch 1310, an LED 1311, an AC-supercap r 1312, a bridgerectifier 1313, and a piezo element 1314. The inset photo 1309 in FIG.13B is showing that the charged AC-supercap can turn on a green LED (theLED running time depends on the stored energy).

The generated pulse from the piezoelectric element was passed throughthe bridge rectifier and fed across the AC-supercap. A suitable Zenerdiode might also be used to clamp the voltage if it is over the 3Vrating of the AC-supercap. A 3 MΩ resistor was used to simulate amicro-power sensor load.

For demonstration purpose, a green light-emitting diode (LED) was alsoused as a high-power pulse load. In FIG. 13C, the generated electricpulses (V_(i)) (plot 1315) from finger tapping of the piezoelectricelement is shown, and the pulse energy was stored into the AC-supercapwith a DC output (V_(o)) close to 2V (plot 1316), which provides aconstant current to the mega ohm low-power load. It can even turn on agreen LED (inset photo 1309) for a short period. In a differentexperiment to simulate applications of environmental energy harvesting,the generated irregular pulses (V_(i)) from a piezoelectric element,which was attached to a motor as the vibration source, were stored intoan AC-supercap after rectification, and the stored energy (or DC voltageV_(o)) was supplied to power a micro-power element.

60 Hz line-frequency low-voltage AC/DC converter was also demonstratedusing the circuit shown in FIG. 14A, having voltage input 1401,full-wave rectifier 1402 and AC-supercap 1403. The AC input signal wasfirst passed full-wave rectifier 1401 and then filtered by AC-supercap1403 before feeding to a load. 60 Hz sine wave with an amplitude of 4.2V from a signal generator (which is shown in plot 1404) was applied tosimulate the AC voltage in this demonstration. The output after passingthe full-wave rectifier when an AC-supercap was not connected (plot1405), and the output with the AC-supercap (plot 1406) are also shown inFIG. 13D. 3V DC voltage was obtained after the ripple voltage wassmoothed by the AC-supercap. These demonstrations confirm the promise ofour CBC based AC-supercaps for ripple current filtering and pulse energystorage.

Several embodiments of the invention have been shown and described indetails. Some other embodiments are:

For the EOG electrode material, one embodiment described for cellulosepaper 101 is Kimwipes® wiper sheets. Other cellulose paper or sheet canbe similarly used.

For the EOG electrode material, one embodiment described that EOG isdeposited on the carbonized cellulose sheet using microwave plasma CVD.Other plasma deposition method can also be used.

For the AC-supercaps assembled using EOG electrode material, oneembodiment described KOH based aqueous electrolyte is used. Many otheraqueous electrolytes and many other organic electrolytes can also beused.

For the AC-supercaps, one embodiment described 1 M tetraethylammoniumtetrafluoroborate (TEABF4) in anhydrous acetonitrile (AN) solution isused as organic electrolyte. Many other salts in many other anhydrousorganic solvents with different concentrations can also be used as theorganic electrolyte.

For the carbon nanofiber network (or nanoweb) (CNN) electrode, oneembodiment described bacterial nanofiber cellulose aerogel is used forplasm pyrolysis to obtain the CNN electrode material. Other cellulosenanofiber networks or aerogels, such as those cellulose nanofibersextracted from plants or other biomass can also be used for plasmapyrolysis. Furthermore, in addition to cellulose polymer nanofiber,other synthesized polymer nanofibers such as polyacrylonitrile (PAN),polyimide (PI), and phenolic resin nanofibers can also be used forplasma pyrolysis to obtain carbon nanofiber network as electrode. Thepolymer nanofibers can be derived from their solution by differentmethods, such as through electrospun.

For AC-supercap cell assembling, one embodiment was described that twosymmetric electrodes, separated by a separator that is soaked withaqueous or organic electrolyte, are assembled into a coin cell package.As known in the area of AECs, conventional supercapacitors andbatteries, electrodes can also be wound up to pack into cylindricalformat. They can be stacked up to pack into prism format, or many otherpackage shapes.

While embodiments of the invention have been shown and described,modifications thereof can be made by one skilled in the art withoutdeparting from the spirit and teachings of the invention. Theembodiments described and the examples provided herein are exemplaryonly, and are not intended to be limiting. Many variations andmodifications of the invention disclosed herein are possible and arewithin the scope of the invention. Accordingly, other embodiments arewithin the scope of the following claims. The scope of protection is notlimited by the description set out above.

The disclosures of all patents, patent applications, and publicationscited herein are hereby incorporated herein by reference in theirentirety, to the extent that they provide exemplary, procedural, orother details supplementary to those set forth herein.

Concentrations, amounts, and other numerical data may be presentedherein in a range format. It is to be understood that such range formatis used merely for convenience and brevity and should be interpretedflexibly to include not only the numerical values explicitly recited asthe limits of the range, but also to include all the individualnumerical values or sub-ranges encompassed within that range as if eachnumerical value and sub-range is explicitly recited. For example, anumerical range of approximately 1 to approximately 4.5 should beinterpreted to include not only the explicitly recited limits of 1 toapproximately 4.5, but also to include individual numerals such as 2, 3,4, and sub-ranges such as 1 to 3, 2 to 4, etc. The same principleapplies to ranges reciting only one numerical value, such as “less thanapproximately 4.5,” which should be interpreted to include all of theabove-recited values and ranges. Further, such an interpretation shouldapply regardless of the breadth of the range or the characteristic beingdescribed.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which the presently disclosed subject matter belongs.Although any methods, devices, and materials similar or equivalent tothose described herein can be used in the practice or testing of thepresently disclosed subject matter, representative methods, devices, andmaterials are now described.

Following long-standing patent law convention, the terms “a” and “an”mean “one or more” when used in this application, including the claims.

Unless otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.” Accordingly, unless indicated to the contrary, thenumerical parameters set forth in this specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by the presently disclosed subject matter.

As used herein, the term “about,” when referring to a value or to anamount of mass, weight, time, volume, concentration or percentage ismeant to encompass variations of in some embodiments ±20%, in someembodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, insome embodiments ±0.5%, and in some embodiments ±0.1% from the specifiedamount, as such variations are appropriate to perform the disclosedmethod.

As used herein, the term “and/or” when used in the context of a listingof entities, refers to the entities being present singly or incombination. Thus, for example, the phrase “A, B, C, and/or D” includesA, B, C, and D individually, but also includes any and all combinationsand subcombinations of A, B, C, and D.

1-32. (canceled)
 33. An AC-supercapacitor comprising: (a) a pair ofcarbon nanofiber network (CNN) electrodes; (b) a separator to separatethe electrodes; and (c) electrolytes between the electrodes, wherein theAC-supercapacitor is operable for running at frequencies of at least 0.1kHz.
 34. The AC-supercapacitor of claim 33, wherein theAC-supercapacitor is operable for running at frequencies of at least 1kHz.
 35. The AC-supercapacitor of claim 33, wherein theAC-supercapacitor uses aqueous electrolyte.
 36. The AC-supercapacitor ofclaim 33, wherein the AC-supercapacitor uses organic electrolyte. 37.The AC-supercapacitor of claim 33, wherein the AC-supercapacitor has avoltage operation window of at least about 2 V. 38-58. (canceled)